With respect to target designation, in the past, laser target designators have been utilized to designate a particular target by directing laser radiation at the target. This radiation is reflected by the target in all directions and is detected by a guided bomb, missile or projectile which locks onto this radiation. The radiation from the target then permits the internal guidance system for the missile, projectile or bomb to home in on the target designated by the laser.
One of the problems when utilizing such a target designator, is the problem that radiation from the laser may impinge upon objects near the target such as grass, cement, or trees, etc. Additionally, some of the radiation reflected from the target may impinge on nearby grass or cement. In either case the guided projectile may home in on an object removed from the target.
With respect to the reconnaissance application of the subject invention, the finding of a man-made object amidst foliage has been a continuing reconnaissance problem. In the past, light beams have been swept across a given area and light reflected from a shiny object is detected to indicate the presence of a man-made object. However, foliage and other naturally occurring objects also reflect light, making it difficult to pick out a man-made object in this environment.
The present invention solves the problem of detecting radiation returned from man-made objects by recognizing that most man-made objects are specular. If polarized light is projected toward an object, if the object is specularly reflecting the polarization will be preserved. However, if the object is non-specular, polarization is destroyed. The subject invention takes advantage of this phenomenon to distinguish man-made objects from naturally occurring objects. In the system described herein, the polarized light is produced by a laser which produces two pulses (a doublet) with a predetermined interpulse spacing and with the radiation making up one of the pulses polarized in one direction, while the radiation making up the other pulse is polarized in an orthogonal direction. It has now been discovered that due to a thermal birefringent effect, radiation produced by a laser rod is inherently orthogonally polarized due to thermal stress which occurs when the laser rod is excited by a flash lamp. Advantage is taken of this new-found effect by providing the laser with two laser cavities each tuned to light of a different polarization and by providing a Q-switch in each cavity. When these Q-switches are sequentially actuated two separate pulses are produced, each with a different plane of polarization. As will be seen, the two laser cavities enhance the birefringent effect such that two successive laser pulses are produced with different and distinct polarizations.
When this laser is utilized, the initial polarization is preserved by the specular reflection of the man-made object such as a tank, halftrack, armored vehicle, rifle barrel, belt buckle, etc. However, such objects as cement, grass, trees, and other foliage, etc., are non-specular such that laser radiation impinging on these objects is returned with the returned radiation being randomly polarized.
To distinguish between polarized and unpolarized radiation the subject invention includes a detector which responds only to polarized light and rejects randomly polarized radiation such that when this detector is utilized in combination with the aforementioned laser, a system is provided which distinguishes between radiation from a specular object and radiation from a non-specular object. In one embodiment, the laser is set up so that the pulses are respectively vertically and horizontally polarized. The returned radiation is detected by two detectors respectively having a vertically polarized element in front of one detector and a horizontally polarized element in front of the other detector. The outputs from these detectors are added differentially at a differential amplifier such that radiation from non-specular objects at which initial polarization is destroyed results in equal outputs from both detectors which cancel when differentially added. On the other hand, returns from a specular object which preserves the initial polarization results in an output from only one detector at a time and thus a nonzero output is available from the differential amplifier to indicate the presence of a specularly reflecting object. The output of the differential amplifier is applied to a delay circuit having a delay equal to the expected inter-pulse spacing. The signals at the input to and the output of the delay circuit are applied respectively to the input terminals of a two input terminal AND gate, with the output of the AND gate indicating the presence of two pulses having not only the requisite inter-pulse spacing but also the requisite polarization thereby permitting recovery of signals from specularly reflecting objects while rejecting randomly polarized radiation returned by non-specular objects.
It is therefore an object of this invention to provide a system for distinguishing specular from non-specular objects.
It is another object of this invention to provide an improved laser target designation system.
It is a further object of this invention to provide an improved reconnaissance system.
It is another object of this invention to provide a system in which polarized laser pulses are transmitted to a target and in which radiation returned from the target is detected by two detectors which respond to returned radiation polarized in different directions.
It is a still further object of this invention to provide a system which utilizes the thermal birefringent property of an optically pumped laser rod and the difference between specular and non-specular objects in the provision of a laser designation system which rejects background radiation returned from non-specular objects.
These and other objects of this invention will be better understood in connection with the following description in view of the appended drawings in which: